Hi @JoeMurray - you may be able to try out the community built matterbridge if you're able to set up incoming and outgoing webhooks in Mattermost and Slack. I believe it's set up per channel though, so it might depend on how many Slack channels you're in whether or not it's worth setting up.
The notifications will work, but comment threads won't really since Slack doesn't support comment threads (so no one will be commenting on your messages).